A redox flow battery (RF battery) is a large-capacity storage battery that stores new energy obtained by solar power generation, wind power generation, or the like. The RF battery performs charging and discharging using the difference in oxidation reduction potential between ions contained in a positive electrode electrolyte and ions contained in a negative electrode electrolyte. As shown in an operating principle diagram of FIG. 6 for an RF battery 1, the RF battery 1 includes a cell 100 which is separated into a positive electrode cell 102 and a negative electrode cell 103 by a membrane 101 across which hydrogen ions are transported. The positive electrode cell 102 contains a positive electrode 104 and is connected via ducts 108 and 110 to a positive electrode electrolyte tank 106 that stores a positive electrode electrolyte. Similarly, the negative electrode cell 103 contains a negative electrode 105 and is connected via ducts 109 and 111 to a negative electrode electrolyte tank 107 that stores a negative electrode electrolyte. The electrolytes stored in the tanks 106 and 107 are circulated within the cells 102 and 103 by pumps 112 and 113 during charging and discharging. In the case where charging and discharging are not performed, the pumps 112 and 113 are stopped, and the electrolytes are not circulated.
The cell 100 is usually formed inside a structure referred to as a battery cell stack 200 shown in FIG. 7. The battery cell stack 200 is configured such that a layered structure referred to as a sub-stack 200s is sandwiched between two end plates 210 and 220 and fastened with a fastening mechanism 230 (in the configuration shown in the drawing, a plurality of sub-stacks 200s are used). As shown in the upper portion of FIG. 7, the sub-stack 200s has a configuration in which a cell frame 120 including a bipolar plate 121 integrated into a frame 122 shaped like a picture frame, a positive electrode 104, a membrane 101, and a negative electrode 105 are stacked in this order, and the resulting stacked body is sandwiched between supply/discharge plates 190 (refer to the lower portion of FIG. 7). In this configuration, a battery cell 100 is formed between the bipolar plates 121 of the adjacent cell frames 120.
In the sub-stack 200s, circulation of the electrolytes into the cell 100 through the supply/discharge plates 190 is performed by using liquid supply manifolds 123 and 124 and liquid discharge manifolds 125 and 126 which are provided on the frame 122. The positive electrode electrolyte is supplied from the liquid supply manifold 123 through a channel formed on one surface side (front side of the sheet) of the frame 122 to the positive electrode 104, and is discharged through a channel formed on the upper part of the frame 122 to the liquid discharge manifold 125. Similarly, the negative electrode electrolyte is supplied from the liquid supply manifold 124 through a channel formed on the other surface side (back side of the sheet) of the frame 122 to the negative electrode 105, and is discharged through a channel formed on the upper part of the frame 122 to the liquid discharge manifold 126. Ring-shaped sealing members 127, such as O-rings and flat packing, are disposed between the individual cell frames 120 so that leakage of the electrolytes from the sub-stack 200s can be prevented.
The input and output of electric power between the battery cell 100 provided in the sub-stack 200s and an external device are performed by a current-collecting structure using current collector plates composed of an electrically conductive material. A pair of current collector plates are provided for each sub-stack 200s. The current collector plates are electrically connected to the corresponding bipolar plates (hereinafter, referred to as the end bipolar plates) 121 of cell frames 120 located at both ends in the stacking direction among a plurality of cell frames 120 stacked.
In the RF battery, the electrolytes are circulated during charging and discharging. However, circulation of the electrolytes is stopped when charging and discharging are not performed. Accordingly, the pressure in the battery cell 100 changes, and in some cases, the electrical connection between the current collector plate and the end bipolar plate 121 may become insufficient because of the change in pressure. As a technique that overcomes this problem, for example, Patent Literature 1 discloses a technique in which a cushion layer (cushion member) which is deformable in the thickness direction is provided between a current collector plate and an end bipolar plate 121, and a metal layer is disposed on the cushion member side surface of the end bipolar plate 121. Patent Literature 1 describes that preferably, a tin-plated copper mesh is used as the cushion member, and the metal layer is formed by thermal spraying of tin.